Q: My question is, how come that GAMS (MINOS) declares the result as being feasible whereas in reality the solution is not feasible?
GAMS/MINOS by default scales the model, solves the scaled model to optimality and then unscales the model and reports the solution. It can (rarely) happen that the unscaled model is infeasible. If you use anther LP solver (like CPLEX, XPRESS, or one of the free COIN solvers) for your model you will not have this problem.
The quick solution is to use a MINOS option to suppress scaling, then the model solves just fine.
Create a file
minos.opt with the following content:
and then set
optfile=1 somewhere on the command line or set it in your model.